1. Field of the Invention
The present invention relates to a thermal head, and more particularly to a long-spanning thermal head constructed by combining a plurality of head substrates matching with the alignment direction of heating resistance elements arranged linearly on head substrates.
2. Description of the Prior Art
FIG. 1 is a sectional view of a typical conventional thermal head. For example, when thermal printing is attempted in the longitudinal direction of recording paper of A2 format of Japan Industrial Standards as the principal scanning direction, about 600 mm is required as the overall length of the thermal head 1, that is, the printing with W1 in the principal scanning direction. It is difficult to form a small heating resistance element 5 with uniform temperature characteristic over 600 mm on a single head substrate made of, for instance, ceramic. Hitherto, therefore, for example, two head substrates 2, 3 having a printing width W2 of about 300 mm are combined.
Such head substrates 2, 3 are composed of, for example, ceramic, of which coefficient of thermal expansion is EQU .alpha.A=0.73.times.10.sup.-5 .multidot.C.sup.-1 . . . ( 1)
Such head substrates 2, 3 are individually affixed to cooling plates 6, 7 in a size corresponding to the head substrates 2, 3 through a soft adhesive layer 4. The cooling plates 6, 7 are composed of a metallic material excellent in thermal conduction, and in the case of aluminum, the coefficient of thermal expansion is EQU .alpha.B=2.37.times.10.sup.-5 .multidot.C.sup.-1 . . . ( 2)
The end face 2a at the head substrate 3 side of the head substrate 2 and the end face 6a of the cooling plate 7 side of the cooling plate 6 are composed flush, and the end face 3a of the head substrate 2 side of the head substrate 3 and the end face 7a of the cooling plate 6 side of the cooling plate 7 are also formed flush.
Such cooling plates 6, 7 are fixed in the central position in the principal scanning direction of the cooling plates 6, 7, to support plate 8 made of aluminum or the like by screws or the like. That is, the fixing positions 9, 10 of the cooling plates 6, 7 to the support plate 8 are located at a distance of W2/2 from the central position CN relating to the principal scanning direction of the support plate 8 at the location of the end faces 6a, 7a of the cooling plates 6, 7.
The support plate 8 is made of aluminum, and possesses the coefficient of thermal expansion .alpha.B. Here, the difference in the coefficient of thermal expansion is EQU .alpha.B-.alpha.A=1.64.times.10.sup.-5 .multidot.C.sup.-1 . . . ( 3)
Or the cooling plates 6, 7 and support plate 8 may be made of iron, and the coefficient of thermal expansion .alpha.C of such iron is EQU .alpha.C=1.40.times.10.sup.-5 .multidot.C.sup.-1 . . . ( 4)
and the difference from the coefficient of thermal expansion .alpha.A is EQU .alpha.C-.alpha.A=0.67.times.10.sup.-5 .multidot.C.sup.-1 . . . ( 5)
FIG. 2 is a sectional view for explaining the problems of this prior art. For example, the thermal head 1 changes from ordinary temperature of 25.degree. C. to high temperature to 90.degree. C. along with the thermal printing action, assuming that the cooling plates 6, 7 and support plate 8 are made of iron, the fixing positions 9, 10 of the support plate 8 and the cooling plates 6, 7 are dislocated in the mutually departing directions by the variations x3a, x3b as shown in FIG. 2. On the other hand, the cooling plates 6, 7 fixed in the fixing positions 9, 10 of the support plate 8 is dislocated in the mutually departing direction from the central position CN are expanded by the variations x2a, x2b from the fixed positions 9, 10. The head substrates 2, 3 are similarly expanded by the variations x1a, x1b.
According to the experiment by the present inventor, in such prior art, the results of measurement as shown in Table 1 were obtained.
TABLE 1 ______________________________________ Substrate Cooling plate Support plate x1a x1b x2a x2b x3a x3b ______________________________________ Coefficient of 0.73 .times. 10.sup.-5 1.40 .times. 10.sup.-5 thermal expan- sion (.degree.C..sup.-1) Elongation -71.2 -71.2 -136.5 -136.5 +136.5 +136.5 (.mu.m) ______________________________________
It is known from Table 1 that the interval W3 between the head substrates 2, 3 is extended by EQU W3=x3a+x1a+x3b+x1b=130.6 .mu.m . . . (6)
This extending amount is a length more than 1 dot if the density of the small heating resistance element formed along the principal scanning on the head substrates 2, 3 by the thermal head is 8 dots per 1 mm, that is, 125 .mu.m pitch, and therefore an unprinted white stripe is left over on the recording paper, or a white-out phenomenon occurs. Such white-out seriously lowers the printing quality.
FIG. 8 is a plane view of a head substrate 2. On the head substrate 2, the heating resistance element 5 is formed linearly, and the length W2 along the principal scanning direction of this heating resistance element 5 is the printing width W2. The present inventor measured the degree of warp in the thicknesswise direction (the vertical direction to the sheet of paper in FIG. 8) of the head substrate 2 due to temperature rise at five observation points P1 to P5 at mutually equal intervals. The results of observation are shown in Table 2.
TABLE 2 ______________________________________ Variation of warp due to Cooling plate Temperature temperature change (.mu.m) material change P1 P2 P3 P4 P5 ______________________________________ Aluminum 25.degree. C. 0 -29 -53 -33 0 cooling plate .dwnarw. 90.degree. C. ______________________________________
The mode of change is as indicated solind line L1 in FIG. 9. Generation of such warp is same as in the head substrate 3.
When such head substrates 2, 3 are used, the head substrates 2, 3 are warped due to temperature rise in use, and when the head substrates 2, 3 perform thermal printing, the pressing force of pressing the platen roller to the thermal paper, for example, is not uniform in the principal scanning direction, which may result in uneven printing.